Hi,

It seems that `P.get_minmax_data()`

returns the minmax data corresponding to the plot, not the function.

An option to get a local min of a function would be to use the `minimize`

function, as in

```
sage: f(x) = sin(x)
sage: x0 = 1
sage: xn = minimize(f,[(x0)])[0]
Optimization terminated successfully.
Current function value: -1.000000
Iterations: 3
Function evaluations: 6
Gradient evaluations: 6
sage: f( xn )
-0.999999999995
```

For maximization we could call `minimize`

with `-f`

instead of `f`

.

If minimizing or maximizing one variable functions over an interval we could use `find_minimum_on_interval`

or `find_maximum_on_interval`

respectively. For example

```
sage: (a,b) = (-pi/2,2*pi)
sage: find_minimum_on_interval(f,a,b)
(-1.0, 4.7123889767534486)
```

For more details on these functions you can check the Optimization section of the sage manual.

Note: calling `find_maximum_on_interval(f,a,b)`

with `f`

as defined above yields an error, this might be a bug. A workaround to this is to define `f`

with

```
sage: f = lambda x: sin(x)
```

By the way, what is the correct way to know the global xmin,xmax,ymin and ymax of a function ? A 3-digit approximation is enough. Up to now I'm using get_minmax_data, but is it the correct way ?